It is possible to post activity to a previous day, but not from the app. On any progress email, you can reply with activity for that day.
For example, if you wanted to add activity for August 1, 2015, you would find the progress email for August 1 and respond to it with your tasks.
Format your email with one task per line.
If you didn't receive an email or you've deleted it, you can send an email to email@example.com with the subject line in this format: Progress for Monday, August 1st